Matrices made from polymeric fibers are generally useful filter materials. They are inexpensive, are resistant to a wide variety of chemical and fluid environments, and can be made with particulate removal ratings ranging from about 0.2 .mu.m to more than about 100 .mu.m.
The function of a filter is the removal of suspended particulate material from fluids and the passage of the clarified fluid medium. A filter can achieve fluid clarification by different mechanisms. Particulate materials can be removed through mechanical sieving wherein all particles larger than the diameter of pores of the filter medium are removed from the fluid. With this mechanism, filtration efficiency is controlled by the size of the contaminant relative to filter pore diameter. The efficient removal of very small particles, e.g., less than about 0.1 .mu.m in diameter, therefore requires filter media with very small pore sizes for removal by mechanical sieving. Such finely pored filter media tend to have the undesirable characteristics of high pressure drop across the filter medium, reduced dirt capacity, and shorter life.
A filter may also remove suspended particulate material by adsorption onto the filter surfaces. Removal of particulate material by this mechanism is controlled by the surface characteristics of (1) the suspended particulate material and (2) the filter medium. Most suspended solids which are commonly subjected to removal by filtration are negatively charged in aqueous systems near neutral pH. This feature has long been recognized in water treatment processes where oppositely charged, cationic flocculating agents are employed to improve settling efficiencies during water clarification.
It has been found that, if the surface charges of a particle and the filter sheet are of like sign and have differences in magnitude of greater than about 20 mV, electrostatic repulsive forces will be sufficiently strong to prevent capture by adsorption. However, if the magnitude of the zeta potentials of the suspended particles and the filter surface is small, electrostatic repulsion can be overcome by attractive Van der Waals forces and particles which encounter the filter surface will tend to adhere to the filter pore surface. Filter surfaces characterized by positive zeta potentials, however, are capable of first attracting negatively charged particles to the surface and then strongly retaining them by a combination of attractive electrostatic and Van der Waals forces. Thus, filter materials characterized by positive zeta potentials are capable of efficiently removing, by electrostatic capture, negatively charged particles much smaller than the pores of the filter.
In addition, filter surfaces having a positive zeta potential have the ability to remove many pyrogenic bacterial endotoxins (fever-inducing breakdown products of certain bacterial cell walls) from aqueous systems. A positively charged filter with high dirt capacity would be very useful as a means of purifying water from systems such as wells and storage tanks which can accumulate bacteria and their breakdown products.
There is, therefore, a need for a filter medium having an enhanced capability of filtration of fine particulate matter and bacteria and having a high capacity for removal of pyrogenic matter such as bacterial endotoxins and which, in addition, does not have the disadvantages of undesirable extractable matter. Low levels of total extractable matter are also a great benefit to the pharmaceutical industry as it assures the user that purification by filtration of particulate matter does not result in the introduction of other impurities into the desired product. Low levels of extractables are also important to the microelectronics industry for the production of high resistivity particulate-free ultrapure water.